In the use of a pile hammer for driving a pile into strata, such as the ground, the common practice is to utilize a cushion block, commonly referred to as an impact block or dolly, which is disposed within a drive cap referred to also as an anvil or follower, which is positioned on the upper end of the pile for receiving the impact force of the pile hammer ram during the pile driving operation. This cushion block serves three closely related purposes. First, it reduces metal fatigue by preventing the ram point from striking the drive cap directly. Secondly, it attenuates the driving force of the ram by absorbing a portion of the energy passing through it to the drive cap and pile. Thirdly, it attenuates the rebound force from the pile to the hammer. This rebound force approximates the hammer impact force when the pile approaches refusal driving. In some cases, the reflected shock wave from the strata agrees in frequency with the rebound of the pile to cause a total rebound of approximately twice the input force. These events vary over a period of time from about 0.003 seconds to 0.017 seconds, depending on the type of hammer, the pile material and the soil strata.
As the driving force builds up and the time duration shortens, there is an increase in the rate of energy per second passing through the cushion block. This causes greater deformation and therefore the generation of more internal heat referred to generally as hysteresis heat. Such hysteresis heat is generally defined as the heat generated by the friction between the molecules of a material or gas rubbing on one another when they are compressed or impacted. Such hysteresis heat is developed when compressing air or hammering on metal.
Cushion blocks in use today employ such material as hard wood, micarta, solid nylon discs and the like. These materials all have heat insulating qualities which prevent the heat from being readily dissipated to the metal drive cap for ultimate removal into the ambient air. Therefore, the temperature of the cushion block rises during the pile driving operation and destruction results to the block either by plastic flow or ignition.